1. Field of the Invention
The present invention relates to a valve timing control system for an internal combustion engine, which is for controlling the valve timing of the internal combustion engine.
2. Description of the Related Art
FIG. 13 is a conceptual illustration of a configuration of an internal combustion engine including a valve timing mechanism, disclosed in Japanese Patent Application Laid-open No. 6-299876.
As FIG. 13 shows, an internal combustion engine 1, including a conventional valve timing control system, is equipped with an air cleaner 2, an air flow sensor 3, a throttle valve 4, a throttle sensor 5, an intake pipe 6, an injector 7, an ignition plug 8, an exhaust pipe 9, an O.sub.2 sensor 10, a catalytic converter 11, a sensor plate 12, a crank angle sensor 13, a sensor plate 14, a cam angle sensor 15, an oil control valve 16, an ECU 17 and an ignition coil 18.
Moreover, as shown in FIG. 14, a housing 21, a rotor 22, a retarding chamber 23, and an advancing chamber 24 constitute a VVT (Variable Valve Timing) actuator 20.
The air cleaner 2 is installed at an opening of the intake pipe 6 to clean the air the internal combustion engine 1 intakes. The airflow sensor 3 is installed on the downstream side of the air cleaner 2 to sense the air intake amount into the internal combustion engine 1. The throttle valve 4 is opened and closed in connection with the accelerator pedal to adjust the air intake amount. The throttle sensor 5 detects the opening degree of the throttle valve 4.
In such an internal combustion engine, when the driver steps on the accelerator pedal, the throttle valve 4 opens/closes, so that air is mixed with a fuel injected from the injector 7 and this air-fuel mixture is introduced into cylinders. The fuel-air mixture is then ignited by the ignition plug 8 to push the pistons downward by the combustion thereof, thereby causing the crank shaft to rotate. The rotation of the crank shaft is derived as the output of the internal combustion engine.
With this operation of the internal combustion engine, the O.sub.2 sensor 10 detects the residual oxygen amount in the exhaust gas. The catalytic converter 11 simultaneously removes THC, CO and NOx which are harmful gases.
FIGS. 14 and 15 are enlarged illustrations of an essential portion of the VVT actuator.
In FIG. 14, the VVT (Variable Valve Timing) actuator 20 is situated on the intake side, and is composed of the housing 21, the rotor 22, the retarding chamber 23 and the advancing chamber 24.
The rotor 22 is fixedly fitted on a cam shaft (not shown) to keep a constant positional relationship (for example, the positional relationship shown in FIG. 14) with respect to the housing 21.
A timing belt, timing chain or the like (not shown) is set on the housing 21. This timing belt or the like is also placed on a crank shaft (not shown).
With this structure, the rotation of the crank shaft causes the rotation of the cam shaft through the timing belt or the like.
Furthermore, the oil control valve 16 controls the oil pressure to be applied to the VVT actuator 20 fitted to the cam shaft.
Thus, in order to vary the valve timing in the internal combustion engine, the ECU 17 controls the VVT actuator 20 through the oil control valve 16 to adjust the amount of the lubricating oil to be supplied to the retarding chamber 23 and the advancing chamber 24.
The ECU 17 shifts the relative position of the rotor 22 with respect to the housing 21, for example, from the position shown in FIG. 14 to the position shown in FIG. 15, thereby changing the valve timing.
FIG. 16 is a graph showing the characteristics of the relationship between valve timing-and valve overlap. In this case, the term "valve overlap" signifies the overlap between the time period during which the intake valve is in the open condition and the time period during which the exhaust valve is in the open condition.
For instance, in order to retard the valve timing of the intake valve, the oil control valve 16 supplies the oil to the retarding chamber 23. At this time, the rotor 22 is rotated counterclockwise with respect to the housing 21, and the valve timing of the intake valve is retarded (in the direction indicated by arrow A in FIG. 16), so that the valve overlap decreases.
On the other hand, if the valve timing of the intake valve is advanced (in the direction indicated by the arrow B in FIG. 16), the valve overlap increases.
Furthermore, in the case of retarding the valve timing of the intake valve to a maximum, the housing 21 is brought into contact with the rotor 22 and is fixed at the position (see FIG. 15) where it stops mechanically, this being the position where the valve overlap assumes the minimum value.
In the following description, the advance amount in the case where the valve timing of the intake valve assumes this position will be referred to as a maximum retardation value, and in this case the valve timing of the intake valve is expressed as being at the maximum retardation position.
In the valve timing control for the internal combustion engine, the substantial advance amount (which will be referred to hereinafter as a VVT control variable) by the VVT mechanism is determined with the aforesaid maximum retardation value being employed as a reference. Moreover, this valve timing control is implemented by the ECU 17. The optimal valve timing required for the internal combustion engine varies according to the operating conditions. Therefore, the ECU 17 always controls the valve timing according to the operational conditions.
For instance, a ROM of the ECU 17 retains a two-dimensional map for determining a desired advance amount on the basis of the engine speed detected by the crank angle sensor 13 and the intake amount sensed by the air flow sensor 3.
Thus, the ECU 17 controls the valve timing so that the VVT controlled amount (variable) coincides with the desired advance amount obtained from the two-dimensional map on the basis of the engine speed and the intake amount.
As mentioned above, the desired advance amount is stored in the form of a deviation of the advance amount from the maximum retardation value employed as a reference, and signifies a desired VVT control variable. Accordingly, if the desired advance amount is zero, the ECU 17 carries out control so that the VVT control variable assumes zero, with the valve timing being set to the-maximum retardation side.
Next, a description will be made hereinbelow of a valve timing detecting device.
The sensor plate 12 and the sensor plate 14 are axially fixed on the crank shaft and the cam shaft, respectively. Projections are formed on outer circumferences of the sensor plates 12, 14.
Furthermore, in the vicinity of the sensor plates 12, 14, the crank angle sensor 13 and the cam angle sensor 15 are located facing the outer circumferences thereof, respectively. The crank angle sensor 13 and the cam angle sensor 15 detect as variations of magnetic fields, the variations in the distance between the crank angle sensor 13 and the sensor plate 12 and the variations in the distance between the cam angle sensor 15 and the sensor plate 14, occurring with rotation of the sensor plates 12, 14 respectively.
Thus, with the rotation of the crank shaft and the cam shaft, the sensor plates 12, 14 rotate, and the crank angle sensor 13 and the cam angle sensor 15 sense the projections on the outer circumferences thereof to thereby detect a crank angle and a cam angle, respectively.
FIG. 17 shows examples of output signals from a crank angle sensor and a cam angle sensor. FIG. 17 shows the characteristics of a four-cylinder internal combustion engine.
In an internal combustion engine with the characteristics shown in FIG. 17, the sensor plate 12 axially attached to the crank shaft has a projection between 76 degCA before top dead center (BTDC) and 6 degCA (BTDC) of crank angle. Thus, in such an internal combustion engine, a signal from the crank angle sensor 13 becomes high (H level) at BTDC 76 degCA, while it becomes low (L level) at BTDC 6 degCA.
Furthermore, In an internal combustion engine with the characteristics shown in FIG. 17, the sensor plate 14 set at its axis to the cam shaft has a projection formed to output an H level at a point (.theta.=20 degCA) 20 degCA prior to the switching of the output signal of the crank angle sensor 13 to the H level. Accordingly, in the maximum retardation value, a signal from the cam angle sensor 15 becomes an H level at a point (.theta.=20 degCA) 20 degCA prior to the switching of the output signal of the crank angle sensor 13 to the H level.
From the above, the ECU 17 calculates (obtains) the phase difference between the cam angle and crank angle, that is the advance amount, according to the following equation, on the basis of the time difference between signals from the crank angle sensor 13 and the cam angle sensor 15 and engine speed. EQU .theta.=(Tcrank-Tcam)/(Tcrank[i]-Tcrank[i-1).times.180 (1)
where,
.theta.: phase difference [degCA] between the cam shaft and the crank shaft; PA1 Tcrank: a required time [msec] from when a free running counter starts until an output signal from the crank angle sensor switches to an H level; and PA1 Tcam: a required time [msec] from when the free running counter starts until an output signal from the cam angle sensor switches to an L level.
Moreover, Tcrank[i-1] signifies a value in the previous processing cycle with respect to Tcrank[i].
Since the signal input processing for the phase difference .theta. is conducted through interruption processing in a processing program of the ECU 17, even if the valve timing is on the maximum retardation side, the phase difference .theta. in FIG. 17 is made so as not to assume zero. This is because, if the phase difference .theta. becomes zero on the maximum retardation side, a miscalculation can take place at a minor timing in the interruption processing.
In order to prevent such erroneous calculation processing, a phase difference .theta. is also provided on the maximum retardation side of the valve timing and advance amount at this time is learned as the maximum retardation value. And then the advance control is carried out using this maximum retardation value as a reference.
Thus, the maximum retardation value is learned for the purpose of preventing irregularities in the maximum retardation value due to differences among the accuracies and installation positions of the sensor plates, the cam angle sensor and the crank angle sensor, and those arising from changes with the passage of time. That is, if the maximum retardation value is only stored in a ROM of the ECU 17 and is not actually detected, difficulty is experienced in accurately controlling the valve timing due to the aforesaid differences among the installation positions etc, and this hinders development of the intended performance of the internal combustion engine.
In addition, this maximum retardation value must be learned at a position where the valve overlap is at a minimum.
Regarding the idling condition of an internal combustion engine including the VVT mechanism, the stability of operating conditions is generally considered an important matter, and, through the use of oil pressure, the housing 21 is fixed relative to the rotor 22 at a position where the valve overlap is minimum.
However, in the case where the lubricating oil temperature in the internal combustion engine is increasing, the lubricating oil pressure drops in the idling condition, as compared with that in cold. Consequently, the force whereby the housing 21 is fixed at the position where the VVT control variable becomes zero decreases, and the position of the housing 21 with respect to the rotor 22 is varied by the force from the cam shaft so that there is a possibility of the valve timing advancing.
Accordingly, if the maximum retardation value is learned when the lubricating oil pressure is low, the maximum retardation value may contain errors.
Secondly, a description will be made hereinbelow of the operation of the conventional valve timing control system for an internal combustion engine.
FIG. 18 is a flow chart showing the processing contents of the conventional internal combustion engine valve timing control system.
As FIG. 18 shows, the processing starts at a step 1801 to calculate a phase difference between the cam shaft and the crank shaft on the basis of a rotational phase of the cam shaft detected by the cam angle sensor 15 and a rotational phase of the crank shaft detected by the crank angle sensor 13. The phase difference thus obtained signifies the present advance amount of the cam shaft with respect to the crank shaft.
Subsequently, a step 1802 follows to determine whether or not the operating condition of the internal combustion engine 1 is an idling condition.
If the internal combustion engine 1 is in the idling condition, the flow proceeds to a step 1803 to learn, as the maximum retardation value, an advance amount in the idling condition. Ordinarily, since the VVT mechanism does not operate in the idling condition, the maximum retardation value detected in the step 1803 becomes an advance amount when the valve timing is on the maximum retardation side.
Next, the flow proceeds to the step 1804 to calculate a deviation between the present advance amount detected in the step 1801 and the maximum retardation value learned in the step 1803. This deviation is a VVT control variable obtained by the valve timing system.
Moreover, in the idling condition, since the present advance amount is equal to the maximum retardation value learned in the step 1803, the VVT control variable assumes zero.
The ECU 17 determines a valve timing control variable on the basis of a deviation between the VVT control variable thus detected and a desired advance amount calculated in the ECU 17 in advance. Further, the ECU 17 controls the oil control valve 16 for driving the VVT actuator 20 so that the VVT control variable coincides with the desired advance amount.
As described above, in the-conventional valve timing control system for an internal combustion engine, the phase difference between the cam angle and the crank angle is learned in the idling state where the valve overlap becomes a minimum.
Nevertheless, in the idling condition, after the oil temperature rises, the oil pressure drops, and the force by which the housing 21 is fixed at the maximum retardation value with respect to the rotor 22 becomes weak so that there is a possibility that the housing 21 cannot be fixed at the maximum retardation value by the operating force of the cam shaft. This cam shaft operating force signifies a reactive force from a valve operating when a cam of the cam shaft activates the valve.
In such a case, since a position advanced a certain degree from the maximum retardation value is learned as the maximum retardation value, the valve timing control based upon the VVT control variable is implemented in a state where the erroneous maximum retardation value is used as a reference, thus creating an error between the valve timing control variable and the desired advance amount, making it difficult to develop the intended engine performance.